[Simh] Virtual memory

[Johnny Billquist]

Hi, Larry.

On 2018-02-18 18:20, Larry Baker wrote:
>>
>> On Feb 18, 2018, at 9:00 AM, <simh-request at trailing-edge.com 
>> <mailto:simh-request at trailing-edge.com>> 
>> <simh-request at trailing-edge.com 
>> <mailto:simh-request at trailing-edge.com>> wrote:
>>
>> Date: Sun, 18 Feb 2018 11:08:24 +0100
>> From: Johnny Billquist <bqt at softjar.se <mailto:bqt at softjar.se>>
>> To: Larry Baker <baker at usgs.gov <mailto:baker at usgs.gov>>
>> Cc: SIMH <simh at trailing-edge.com <mailto:simh at trailing-edge.com>>
>> Subject: Re: [Simh] Simh Digest, Vol 169, Issue 58
>> Message-ID: <9314564c-2452-fd85-bc18-075301e9ce04 at softjar.se 
>> <mailto:9314564c-2452-fd85-bc18-075301e9ce04 at softjar.se>>
>> Content-Type: text/plain; charset=utf-8; format=flowed
>>
> 
> Johnny,
> 
>> Memory mapped overlays potentially wasted lots of space. The "problem"
>> is that each overlay level must be page aligned, which means 8K aligned.
>> It also means that you cannot have more than 7 levels, since you only
>> have 8 pages. Compare to disk based overlays, which can pack the levels
>> right next to each other, and it should be obvious that you potentially
>> loose a lot of memory with memory mapped overlays.
> 
> I was confused why you think memory is wasted.  It actually is not.  It 
> is true that the program address space can only be mapped by an entire 
> window.  Thus, you have to be clever about packing the 8K address 
> window.  However, there is no requirement that an entire address window 
> be mapped.  The real memory allocated is only the portion that is 
> mapped.  The system memory is packed tightly, just the same for regular 
> contiguous (on RSX at least) allocations for any program.  Real memory 
> is not wasted.

Oh! We're talking past each other. You are absolutely right. It does not 
waste physical memory space. It potentially wastes virtual memory 
address space. That is what I was referring to. If you want to get a lot 
of code in, which usually is the case if you're dealing with overlays in 
the first place, then memory resident overlays can make it complicated 
to really get more memory out of the thing with memory mapped overlays. 
It can be done, and it certainly is way faster, but there are potential 
headaches in there.

All good, I hope. :-)

   Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol